The Nerve Growth Factor (NGF) gene family promotes neuronal survival and differentiation in the developing nervous system and functions in the mature brain to maintain certain populations of neurons. Basal forebrain cholinergic neurons, which are lost in Alzheimer's Disease, appear to require target-derived NGF throughout their lifetime. Similarly, Brain-Derived Neurotrophic Factor (BDNF) and Neurotrophin 3 (NT3), recently identified members of the trophic factor family, appear to support the survival of basal forebrain neurons as well as nigral dopaminergic neurons, the principal neurons lost in Parkinson's Disease. Thus, neurodegenerative diseases associated with aging may have as an underlying cause either an inability to obtain or respond to members of the NGF family. Even if they do not play a causative role, these factors may be of therapeutic value for rescuing neurons that otherwise would be lost in the course of a neurodegenerative disease. We have recently identified yet a fourth member of the NGF family, named Neurotrophin 4 (NT4), from frog genomic DNA using polymerase chain reaction (PCR) primers directed to conserved amino acid sequences. The goals for Phase 1 are to determine the structure of a rat NT4 genomic clone that has been isolated using the frog NT4 gene fragment as a probe. The rat gene will then be used to determine the pattern of NT4 expression in the CNS and peripheral tissues during development and in the mature animal. Within these tissues, the cell types that contain NT4 mRNA will be identified by in situ hybridization. These results will then be used to form a working hypothesis of the neuronal populations that may require NT4 for their survival during development and/or their maintenance in the adult. The goals for Phase 2 are to identify the neuronal populations responsive to NT4 and describe the biological actions of NT4. Towards this goal, mature NT4 protein will be expressed in the baculovirus expression system and purified to homogeneity. NT4 protein will then be tested on neurons in vitro and in vivo by injection and retrograde uptake of the radiolabeled factor. The purified protein will also be used to raise function-blocking antibodies in order to directly test the survival-promoting effects of NT4 on neurons in immature and mature rats.